Axial air gap generator with cooling arrangement



Feb. 18, 1969 W.KOBER N 3,428,840

AXIAL AIR GAP GENERATOR WI'TH COOLING ARRANGEMENT Filed Jan. 9, 1967 I Sheet INVENTOR.

Feb. 18, 1969 Filed Jan; 9, 1967 W. KOBER AXIAL AIR GAR GENERATOR WITH COOLING ARRANGEMENT Sheet 2 0113 fV/dd/Z/V/ zaaaz INVENTOR.

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AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Sheet 3 of 1s Mad/4M {0552 IN VE N TOR.

Feb. 18, 1969 w. KOBER 3,428,840

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Sheet 4 of 13 2 Z w 57 45 A327 N Z9 w y w d W v E 1] I W/ddM/W K4566 INVENTOR.

Feb. 18, 1969 KOBER 3,428,840

AXIAL'AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Sheet 5 of 15 W/dM/M 150555 Y INVENTOR.

Feb. 18, 1969. w. koBER ,84

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Sheet (9 of 13 Wad/4M [dZ-Z INVENTOR.

Feb. 18, 1969 3,428,840

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 w. KQBER Sheet 95 fig 25 M4 (MM K6555 INVENTOR.

Feb. 18, 1969 3,428,840

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 W. KOBER Sheet 8 of 13 INVENTOR.

Feb. 18, 19 69 KOBER 3,428,840

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Sheet 9 of 13 mad/4M 4 0552 I NVE N TOR.

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W. KOBER AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Feb. 18, 1969 w. KOBER 3,428,340

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967 Sheet of 13 iaww- A 77df/l/fl Feb. 18, 1969 w. KQBER 3,428,840

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT Filed Jan. 9, 1967, sheet A? of 15 W/z/AM [656E INVENTOR.

Afraz/z/fk Feb. 18, 1969 w, KOBER 3,428,840

AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT- Filed Jan. 9, 1967 Sheet 3' of 15 W/zzMM [056K INVENTOR WW V United States Patent 3,428,840 AXIAL AIR GAP GENERATOR WITH COOLING ARRANGEMENT William Kober, Rolling Hills, Calif., assiguor to The Garrett Corporaion, Los Angeles, Calif., a corporation of California Filed Jan. 9, 1967, Ser. No. 608,013

US. Cl. 310-114 Int. Cl. H02k 21/10 38 Claims ABSTRACT OF THE DISCLOSURE This invention relates generally to an electric generator and, more specifically, to an electric generator having an axial gap and an electromagnetically excited field.

In my copending application entitled Alternating Current Generator, Ser. No. 529,603, filed on Feb. 10, 1966 and assigned to the same assignee as this application, there is disclosed an axial gap alternating current generator wherein the field is excited by permanent magnets. Although an axial gap generator, with permanent magnet field excitation, produces a relatively large capacity of electrical power per unit weight and cost, this factor decreases as the size or total capacity of the generator increases. Therefore, in a large size generator, an electromagnetically excited field is more practical and efficient.

An object of this invention is to provide a novel construction for an electric generator with electromagnetic field excitation which makes possible a generator having a relatively large total capacity and of minimum weight and cost. Another object of this invention is to produce a generator with minimum power losses and a relatively high efiiciency.

A feature of this invention is a generator with an electromagnetic field with little or no iron subjected to alternating magnetic flux, thus substantially reducing the power loss, heat, and weight associated with this component, as well as the fabrication-cost and other problems arising therefrom.

Another feature of this invention is a rotatable electromagnetic field that is relatively strong and capable of running at high speeds.

Another feature of this invention is the provision of a novel rotatable field structure for reducing the mass thereof without sacrificing the field strength.

Another feature of this invention is the provision of novel field coils for an axial gap generator for producing an alternating current that resembles a sine wave.

Another feature of this invention is the provision of novel means for cooling the field structure and electromagnetic coils.

Another feature of the invention is the provision of field coils having maximum turns and minimum resistance per unit volume.

Another feature of this invention is the provision of a novel stator for an axial gap electromagnetic alternator which is strong and has maximum efliciency.

3,428,840 Patented Feb. 18, 1969 These and other features and advantages of the invention will be more apparent after a perusal of the follow ing specification taken in connection with the accompanying drawings wherein:

FIG. 1 shows schematically an alternator of the prior art type having a radial gap;

FIG. 2 shows schematically an alternator of the prior art type having an axial gap;

FIG. 3 shows schematically an ironless alternator of the prior art type having permanent magnets;

FIG. 4 shows schematically a side elevation in partial section of an ironless generator having an electromagnetic field excitation in accordance with this invention;

FIG. 5 shows schematically a view of the generator taken on line 55 of FIG. 4 in the directions of the arrows;

FIG. 6 is a pictorial view of one of the rotors of the generator shown in FIG. 4 showing the bare rotor removed from the generator;

FIG. 7 is an exaggerated enlargement (not to scale) of an embodiment of one of the field coil windings shown in FIG. 5;

FIG. 8 is an exaggerated enlargement in cross-section taken on line 8-'8 in FIG. 7;

FIG. 9 is a plan view of another embodiment of a fourpole rotor without the field coils showing the rotor removed from the generator;

FIG. 10 is an elevation in partial section of the rotor in FIG. 9 taken in the direction of arrows and substantially along the broken line 1010;

FIG. 11 is another elevation in partial section of the rotor in FIG. 9 taken in the direction of arrows and substantially along the broken line 1111;

FIG. 12 is a pictorial view of one embodiment of a two-pole rotor removed from a generator and shown without field coils;

FIG. 13 is an axial view of the two-pole motor of FIG. 12;

FIG. 14 is a side view of the rotor of FIG. 13;

FIG. 15 is another side view of the rotor rotated from the view in FIG. 14;

FIG. 16 is a pictorial view of another embodiment of a two-pole rotor;

FIG. 17 is an axial section of a rotor with removable poles;

FIG. 18- is an axial view of the rotor in FIG. 17;

FIG. 19 shows a pictorial view of the rotor of FIGS. 17 and 18 with the poles removed;

FIG. 20 shows a pictorial view of two poles connected by a pole face damper removed from the rotor of FIG. 19;

FIG. 21 shows a pictorial view of the two-pole rotor of FIG. 16 with one of the field coils in place and the other removed;

FIG. 22 is a side elevation in partial section of the twopole rotor of FIG. 21 with both field coils in place;

FIG. 23 is a front elevation of the two-pole rotor of FIG. 22;

FIG. 24 is a cross-section of a pole and part of a rotor showing one embodiment of a field coil;

FIG. 25 is a pictorial view of another embodiment of a field coil;

FIG. 26 is a front elevation of the field coil in FIG. 25 showing the thickness of the ribbon conductor exaggerated;

FIGS. 27a, 27b, 28 and 29 are cross-sections of other embodiments of the field coil;

FIG. 30 shows one embodiment of a rotor that has enhanced cooling;

FIG. 31 is a view of another rotor embodiment with enhanced cooling;

FIG. 32isan axial cross-section of the rotor shown in FIG. 31;

FIG. 33 is a schematic cross-section of a portion of a generator with the field coils and stator removed and showing rotor embodiments with two different liquid cooling systems;

FIG. 34 is a plan view of a field coil embodiment for producing a sinusoidal wave;

FIG. 35 is a transverse section through a generator showing the face of the rotor which uses the coil of FIG. 34;

FIG. 36 shows a stator of the prior art for an axial gap generator;

FIGS. 37, 38 and 39 show a two layer prior art stator at different steps during production;

FIGS. 40, 41 and 42 show an improved two layer stator at different steps during production;

FIG. 43 is a schematic cross-section of another embodiment of a generator;

FIG. 44 shows one embodiment of a stator for producing a substantially sinusoidal wave form;

FIG. 45 schematically shows another embodiment of a stator for a two-pole generator for producing a substantially sinusoidal wave form;

FIG. 46 shows an improved three-phase stator;

FIG. 47 shows a plan view of the stator shown in FIG. 42 with stretching means; and

FIG. 48 shows a side view of the stator in FIG. 47 in partial section.

Before describing the various embodiments that incorporate this invention, a brief resume of the salient features of the prior art alternators will be given. A conventional radial gap generator, as shown in FIG. 1, includes a rotating field 11 which has a source of magneto-motive force (MMF), either permanent magnet or an electromagnetic. The produced flux flows radiall through a small air gap, through iron teeth 12, and returns over a long path through a stationary iron stator 13. Between the teeth 12 is disposed a stator winding 14 in which is produced an electro-motive force (EMF) when the field 11 rotates about an axis disposed perpendicularly to the drawing. Thus, all the iron in the stator 13 is subject to itlternating flux making the hysteresis losses relatively arge.

A modification of the generator of FIGURE 1, using an axial air gap, is shown in FIGURE 2. This type may be viewed as functionally identical with FIGURE 1, but with a changed geometry. Patent No. 2,719,031 points out in detail the construction and advantages of such generator types, particularly when using a permanent magnet field. Here, the principal point to be made is that an annular iron stator 16 is the return circuit for the magnetic flux produced by a rotatable permanent magnet 17. Axially extending iron teeth 18 perform the same function that teeth 12 perform in the machine of FIGURE 1. The machine in FIG. 2 has suitable stator coils 19 placed between the teeth 18.

FIGURE 3 shows an embodiment of a prior art generator that includes no stator iron. Two rotating permanent magnets 21 and 22are arranged so that the poles, on one magnet 21, face opposite poles on the other magnet 22. Thus, the large physical distance between the two opposite N and S poles of FIGURES 1 and 2 is eliminated. The two opposite poles in FIG. 3 face each other across a relatively short air gap in which a stator coil 23 is disposed. The function of stator coil 23 is similar to the stator coil 19 of FIGURE 2. However, coil 23 obtains no support from any stator iron, and so requires other methods of support such as a non-magnetic frame 24 shown schematically. This system has the advantage in that hysteresis losses have been eliminated, because now none of the iron carries alternating flux. This system has also the advantage of providing magnets with twice the length for developing MMF over the magnets in the generator of FIG. 2. In turn, flux leakage paths are reduced. Similarly, in an electromagnet type, the field winding, whether concentrated or distributed, also requires length to develop the required MMF. Thus an electromagnet type generator could be built by splitting the field into two parts and also have the advantage of less flux leakage. Another advantage of having the electromagnets in two systems is that in the larger electromagnet machines, the diameter of the electromagnets and centrifugal stresses, developed in it, are proportionally reduced than in the machine with one electromagnet system.

This invention teaches the use of electromagnets instead of permanent magnets, and the passive flux return path is used to develop the required MMF. Thus, practically all of the stator iron is eliminated, with its weight bulk, energy loss, cooling problem, and cost. This is done by bringing the N-S field producing poles close together and opposed physically, across an air gap. Although there are two magnetic field systems, each field in this invention is smaller than that for the conventional machine using one field system. In addition, the machine of this invention has symmetrical fields, thus simplifying manufacture.

FIGURES 4 and 5 show an arrangement for a 4 pole electromagnet generator having two spaced iron discs or rotors 25 mounted on a rotating shaft 28. Each rotor 25 has, for example, four axially extending lugs or poles 27 on which are disposed field windings 29. The direct current for excitation is supplied from an external source through bushes 31 and slip rings 32, or in any other manner known to the art. The rotors 2-5 and slip rings 32 are suitably fixed to rotate with the rotatable shaft 28.

The electromagnet system has an advantage over permanent magnet types of the prior art, in that the air gap flux density can be taken up to the saturation level of the magnetic iron in poles 27, to produce a generator of small size and weight and high performance.

The field windings may be made of round or rectangular wire. However, in the embodiment shown in FIGS. 7 and 8, each pole 27 is wound with a thin conductive riblbon sheet 33, with an insulating sheet 34 applied to one side thereof and wound with it to provide continuous insulation as shown in FIG. 7. The conductive rib'bon 33 is made of, for example, hard rolled copper or aluminum alloys to give conductivity and strength. For additional strength, all the windings 29 on one rotor are wrapped with high strength non-magnetic wire 36 as shown in FIG. 5, to help support the field structure against centrifugal force.

Referring to FIGURE 6, one of the solid iron rotors 25 is shown in perspective and removed from the machine to illustrate the general construction of the bare rotor with the lugs 27 attached. An axially disposed hole 37 is provided for the shaft 28 which holds the rotors as shown in FIGURE 4. When the field windings 29 are energized, magnetic poles are formed wherein an N-pole is disposed between two S-poles cirournferentially around the rotor, in other words, the N-poles and S-poles alternate. The other rotor has similarly polarized magnetic poles. The rotors 25 are mounted on shaft 28 so that an N po1e on one rotor faces an S-pole on the other rotor.

In FIG. 4, the field windings 29 are shown as placed as close to the air gap as is conveniently possible, This will minimize flux leakage. However, there is always some loss due to leakage between the free end of one lug 27 and the portion where the lug 27 joins to the rotor 25. When the lug 27 has a uniform cross-section, as shown, the free end of lug 27 or pole face will have less flux density than the remainder of the lug. A generator should have its most dense flux at the pole face and should have a magnetic flux path with as low a reactance as possible for reasons well-known in the art. The iron of the pole 27 naturally increases the reactance, as does all of the iron in the rotors 25. However, the reactance effect of a given volume of iron varies, being greatest for the parts nearest the coil 29, and diminishing as the distance from the coil increases. Also, the reactance effect decreases as the iron approaches saturation, because its permeability decreases greatly.

Thus, to keep the reactance low, the poles should 'be cross-section, the pole face is less than saturated, due to flux leakage, a explained above. Thus one feature of this invention is that the poles should be tapered so that the cross-section at the pole face is less than the cross-section of the rest of the pole, so that the flux density is greater at the face than at any other point in the pole displaced from the face. Referring to FIGS. 9, and 11, a fourpole rotor 39 having another configuration is shown, As in rotor 25, on one axial end of the rotor 39 are placed poles 40. The outer surface of the poles 40* has a conical shape 41 so that the free end on the face of the poles 40 has the least cross-section.

As above explained, flux saturation of the rotors has a beneficial effect of lowering the reactance as well as the obvious harmful effect of dissipating some of the effective ampere turns of the field coil. In the portion of the rotor which is not under the field winding, flux saturation produces little benefit, and still causes loss of ampere turns, especially since the length of magnetic path in this region is relatively long. Thus another feature is to increase the cross-section in this region, sawing ampere turns with a modest increase in weight.

Thus, the cross-section of the magnetic flu-x path should increase continually from the pole face to the neutral boundary with the next pole face. To produce this result with a minimum of iron, each flux path should be as short as possible and still include the necessary length for the field windings without deteriorating the mechanical strength of the rotor. Another conical section 42 (FIG. 11) on rotor 39 opposite the poles 40, contributes in shortening the flux path behind the poles 40. The re moved material was able to transmit magnetic flux only circumferentially, and the ideal path is a chord from the N-pole to S-pole, which path is obviously shorter. The optimum shape of the rotor 40' to attain the desired results in a four-pole generator, will now be described. However, the same principles hold for rotors having any number of poles greater than two. As mentioned before, the conical section 41 on the outer diameter of the poles 40 reduces the cross-section of the poles as they approach the air gap face. The poles 40 are shown dotted in FIG- URE 9 to indicate their location and shape relative to the novel shape of the rotor 39. The sharp corners on the poles 40 are rounded for conveniene of fitting the field coil windings. Note also that the flux from the pole base (the portion next to the rotor 39) has three paths to exit, one each across the interpole regions 43 (FIG. 9), and one across the hub 44. This hulb path is normally sufficient to supply the extra cross-section behind the poles that is desired, so that it is only necessary to maintain the same areal cross-section at the base of the pole in the areal cross-section of interpole regions 43 between two poles.

This is accomplished, as shown in FIGS. 9 and 11, by shaping the rotor 39 so that two radii 47a and 47b sweep, as shown, having centers on the line where the poles 40 meet the rotor 39. In turn, the cross-section at the center of the interpole region 43 is substantially constant joining the two arcuated portions. This shaping is shown by four deep 'valleys 48 extending radially (FIG. 9). It is obvious that the height and sloping angle of the conical surface 42 is determined 'by the sweeping arcs formed by radius 47a. Thus a considerable amount of useless iron and weight is removed. The general rule for maintaining a constant cross-section of flux path in rotor 39 is to rotate the area of one half the pole base about a pole edge formed at the interpole space 46, and the cross-section is translated in a straight line opposite the interpole space 46 till it joins the ajacent cross-section that was similarly formed. The inside diameter huib path 44 adds the additional desired cross-section to the flux path to insure non-saturation in the rotor 39. The shaping of the rotor 39 can [be obtained by machining, forging, casting or assembled parts which have been brazed, welded, etc. to each other. I

The material of the rotor 25 or 39 is normally pure iron, that is commercially termed, ingot iron, as this iron is inexpensive, has a relatively high saturation level and a relatively low magnetic reluctance. However, when high speeds are contemplated, the rotors may be made of high strength steel, with some regard to magnetic properties. Where high performance is especially important, the rotor may be made of an iron-cobalt alloy which has a satura* tion level higher than ingot iron. In addition when high speeds are to be used, mechanical problems would supersede the above flux path consideration. Any shape may then be used, provided it equals or exceeds the above flux path requirements. High speed construction would generally tcnd toward a hub, which has a relatively longer axial length than the hub 44 shown in FIG. 10 to provide a relatively long shaft hole 37. A high speed design for a rotor may have a shape as shown in FIGURE 6, wherein a frusto-conical section 25a, behind the poles 27, may be exaggerated into a nearly complete cone, so that the top of the frusto-conical section is truncated at the shaft hole 37.

The contribution to electrical output of the generator of the flux density at a point on a pole face varies directly as the flux density times the velocity. For a ring-shaped pole element with a fixed radius and an infinitesimal width, the flux contribution is proportional to the radius of the area of this ring element. Since the electrical output is proportional to flux density times velocity, the electrical output as produced by the ring-shaped element is related to the square of the radius of the element. Thus any flux in a pole located near the center of rotation produces relatively little electrical power, and, therefore, the inside of the diameter may be made relatively large to accommodate the field coils :and shaft without substan tially detracting from the total output of the generator.

The above description describes the preferred novel shape for a four or more pole rotor. Now the preferred embodiment for a two-p0le rotor will be described. Referring to FIGURE 12, a two-pole rotor 53 is shown removed from the generator. The rotor 53 has an N-pole 54 and S-pole 55 with an interpole slot 57 therebetween. The central enlargement of the interpole slot 51 is required so that the field coils (to be described hereinafter), will not interfere Wit-h the shaft which fits within a hole 58, more clearly shown in FIGURE 13.

In this embodiment the required reduction in crosssection for each pole, from the attached end to its free end at the air gap, is provided as before by a conical section 59. The required cross-section for the rotor 53 is obtained by rotating the areal crosssection of the attached end of one of the poles about a center 64 (FIG. 14). The desired increase in area in this region over that of the attached end of the pole is secured by displacing center 64 from bottom of the slot 57 whereby the distance between center 64 and the bottom of the slot 57 furnishes an added flux path between the N-pole and S-pole. As before the extra cross-section is important to avoid loss of field MMF due to a reluctance drop. Another way of adding flux cross-section in the rotor 53 would be to leave relatively large fillets (not shown) at the bottom of the interpole slot 57 next to the rotor. Only a slight cross-section area for the field coils would be given up, and the fillets would add a minor problem of shaping the field coils. Therefore, a considerable portion of the interpole slot 57 should have walls which are axially parallel to avoid the tendency of the field coils coming out of the slot 57. The shape of the rotor 53 can be best explained by describing one method of manufacturing the rotor.

The rotor is made from a cylindrical iron billet with the required outer diameter. The interpole slot 57 with the required relief for the field coils and the shaft is cut at one end. The hole 58 for this shaft is drilled on the axis. The other end of the billet is milled to form a semi cylindrical surface 68 which has the same diameter of the billet and whose axis is disposed parallel to and spaced from the bottom of slot 57.

Referring to FIGURE 16, there is shown another embodiment of a two-pole rotor 60, which is similar to the rotor 53 except the rotor 60 does not have the outer conical surface 59 (FIGURE of rotor 53. Thus the required reduction in cross-section for each pole 69 is provided by steps 61, 62 and 63. The steps may somewhat complicate the field-coil shape but, as will be shown hereinafter, the problem is minor. A continuous slope may be used instead of steps; then means need to be provided to retain the field coils on the rotor. Similar steps may be applied to rotors with four or more poles to supplement or replace the conical surface.

Referring to FIGURES 17, 18 and 19, there is shown an embodiment of a rotor 71 which could run at speeds where mechanical strength is not a primary problem. Although the rotor 71 is shown with four poles 74, it could have any number of poles. The other end of the rotor 71 opposite the poles 74 could have any of the shapes already described, thus that portion is not shown. The end of the rotor 71 where the poles are attached has formed thereon a plurality of concentric buttress type grooves 73 to provide strength for counteracting the centrifugal force developed by the poles. Also the buttress type grooves provide added contact area to overcome the effect of small unavoidable air gaps between mating surfaces. The pole pieces 74 have mating grooves at one end and have one or more tapped holes 76 (FIG. 17) which receive suitable bolts 75 passing through the rotor 71 through suitable holes to hold the pole piece 74 against the rotor 71. The bolts 75 are preferably of magnetic material to avoid loss of flux section. When extra strength is required, the rake angle on the grooves 73 could be made sharper (not shown) to hold the pole piece more firmly in place when running, and thus help relieve the bolts from destructive stresses. When speeds are low, one groove will suffice or even none, depending on the bolts alone to counteract the centrifugal force. Because the pole pieces 74 are detachable, the generator would have the advantage of having its poles made of cobalt steel with a higher flux saturation level than ingot iron. The rest of the rotor 71 can be made of ingot iron as cobalt steel is relatively expensive.

The rotor 71 having removable pole pieces could more readily accommodate a pole face damper than, for example, the rotor 53 with fixed pole pieces. Referring to FIG. 20, a pole face damper 79 is shown installed on two poles 80 for a two pole generator. The two poles 80 have their faces radially grooved to hold damper bars 82. The bars 82 connect the outer ring 83 to inner ring 84. The interpole slot is covered by a larger bar 86, also joined electrically and mechanically to rings 83 and 84. The field coils (not shown) could be mounted around the poles 80 behind the damper 79 and the poles, in turn, attached to the rotor 71 (FIGURE 19). Since the pole face damper 79 can be placed on the poles 80 without the field coils in place, the damper 79 may be brazed to the poles 80. Thus added performance and ease of manufacture is attained. The damper 79 also holds the field coils in place. A damper 79 such as that of FIGURE 20 could be added, for example, to the rotor body 53 (FIGURE 12) having fixed poles, if the poles are :pre-slotted and the field coils first placed in position around the poles. The damper 79 would then be mounted on the poles and held in place by spot welding or other methods which do not require destructive temperatures so as not to damage the field coils. The damper inherently reduces the pole face cross-section, but it does ensure high flux saturation in the remaining pole face iron. However, an alternate method would be to insert cobalt steel within the openings of the damper 79 and then to attach the assembly to the pole pieces. In this manner the same high total flux level is maintained in spite of the lost area in the pole face taken up by the damper.

In a generator according to the invention, the efficient disposition of the field windings presents many new problems. The generator capacity varies as the square of the total flux, and the field coil heat losses vary as the square of the field current. Thus, until the poles become saturated with magnetic flux, capacity increases, and electrical efficiency remains constant. Thus, the advantage of reaching flux saturation level at the pole face is unquestionable. However, by going beyond saturation, more capacity is available, but at a loss of efficiency and an increase in heat. At this high flux level, the poles have a very low differential permeability, and the reactance of the generator decreases sharply. Thus, much overload capacity becomes available, particularly at low power factors.

To secure a balanced design, the air gap is chosen so that the field winding losses are in the region of one to three times the stator winding losses. To balance the weight vs. performance, the field coils are made much bulkier than the stator coils. Because of this, and also since the main function of the field is to span the large air gap, the ampere turns in the field are much greater than those in the stator. Thus, the stator armature reaction causes no major disturbance to the field pattern. As the saturation of the poles advances at overloads, the disturbing effect of armature reaction gets smaller.

FIGURE 21 shows the two-pole rotor body 60 of FIG- URE 16 with one field coil 87 in place. The coil 87 is made of wide thin copper ribbon. The width of the ribbon varies so that when coil 87 is wound the coil 87 fits the steps 61, '62 and 63 in the slot as shown in FIG- URE 22. However, the outside portion of the coil 87 is smooth, as shown. When a second field coil 88 is added, as shown, in FIGURE 23, the coils should touch along the center line of the slot 66, except in the center, where room will be left for the shaft 28. If the slot 66 is made wider, the coils 87 and 88 can have more cross-section, but area will be lost on the poles 69. If the slot is made deeper, the coils gain cross-section and become longer, but the longer coils will have somewhat more flux leakage. In addition the pole length will also increase, and in turn the weight.

With the relative balance of rotor body and field coils established, the field energy for pole flux saturation, can be readily calculated for any assigned spacing of the opposing poles of the rotor body. A stator can then be designed to fill this space, with due allowance for clearance. At the assigned full load, the stator coil loss can then be calculated. An alternate design can be made 'by changing the air gap. If the air gap is smaller, the field loss will reduce as the square of the air gap, and the stator loss will increase linearly, while still providing for the mechanical clearance between pole faces and stator. The choice of an exact ratio of field loss to armature loss obviously depends on many complex factors. The total loss of stator coils and field coils is now known and is properly balanced and it is checked with available cooling, and the desired generator efliciency. If the loss is too great, a larger scale for the machine is indicated, and vice versa.

The centrifugal force on the field coils 87 and 88 when running is primarily carried by the respective poles 69. However, within the field coils and in the section of conductor within the slot there is developed a tensile stress in the conductor from the center to the outside of the slot similar to that in a bar rotated at its center at right angles to its length. In the generally circular region of the field coils around the outside of the rotor body, the tensile stress formed therein is like that formed in a ring or hoop. The field coils may be made of soft copper to work-hardened copper and also of soft to work hardened aluminum in upward speed progression. If the stresses are too great for the desired coil material and for the pole piece, the coils 87 and 88 can be supported as described before by a wire 36 (FIG. 5) or glass cord or other binding material as shown in FIGURE 23 by a ring 89. This ring 89 may be made of solid metal, such as non-magnetic steel or aluminum. To assist in cooling, the ring 89 is provided with circumferential grooves 91 as shown in FIG. 22. If additional cooling is required axially aligned slots (not shown) could be also formed on ring 89.

Obviously, the field coils when mounted on the rotor have the largest diameter of any part of the rotor. This can be reduced by redistribution of the copper forming the coils. In FIGURE 24 one pole 90 of a typical rotor is shown in section. The cross-section 93a of a coil 93 between the shaft 28 and the pole 90 is shaped thicker and narrower than the cross-section 93b of the coil 93 on the outside of the rotor. The field, coil 93 is preferably bevelled as shown to give more space and cooling area to the stator and its supports. This method of forming the field coil reduces the average overall rotor diameter and increases the cooling area of the coil at the surface, where the area is in contact with the atmosphere. Slightly more flux leakage results, but this is less harmful on the outside of the pole than in the slot, where the opposite pole is close. If the poles have an outside conical surface 59 as shown in FIGURE 14, the coil 93 would be also tilted to fit the conical surface closely, thus also reducing the diameter of the coil in the area near the stator.

Referring to FIGURES 25 and 26, if a single width of relatively thin ribbon conductor 95 is spirally wound one layer over the other to form a field coil 96, the coil could be shaped to have optimum performance. Optimum performance is attained by removing from the coil 96a portion 97 having a shape as shown in FIG. 25. Obviously the uninsulated area of the coil 96 is reinsulated. A major portion of the winding length in coil 96 has much more cross-section, and the overall resistance of the field coil 96 is greatly reduced, while only slightly changing the rotor bodymagnetic characteristics. Coil 96 has also a greater cooling area available in an outer region.

Referring to FIGURE 27a, a ribbon conductor 94 may be disposed radially in a field coil 92. In this form, the width and therefore the cross-section of the ribbon conductor 94 is readily increased on the outside of the pole 90 but at the expense of overall diameter. However, near the stator, the diameter is reduced, as shown. One way to make such a coil 92 is to cut it out of metallic sheets and butt weld or braze successive sheets in a helix. Insulation 91 is provided between the layers of ribbon conductor 94. To provide added cooling means a field coil 98, in FIG. 28, has alternate layers of ribbon conductor 94 made longer than adjacent layers. Naturally insulation 91 is also provided between layers of the ribbon conductor 94. Another method for obtaining enhanced cooling is to spread the layers of ribbon conductor 94a on the outside of the pole 90 to provide air spaces therebetween, as shown in a coil 99 in FIGURE 29. This secures exceptionally good cooling over most of the outside region, but of course, the layers of ribbon conductor 94 are placed in close formation within the interpole slot. To reduce the overall diameter of the assembled rotor, the rotor 90 could be wound with ribbon conductor 94' that are disposed at an acute angle with the axis of rotation as shown in FIG. 27b. A field coil 92 is formed that has an overall diameter less than coil 92 in FIG. 27a. The conductors 94' have suitable insulation 91 and are sloped in the outer circumferential region away from the air gap to allow for more clearance and ventilation for the stator (not shown).

Although means are provided for cooling the field coils, much of the heat generated in the field coil reaches the rotor since the field coils are adjacent thereto. Since the rotor is a massive metallic part, it is an excellent heat sink for the field coils and for the damper 79 (FIG. 20). Thus, much can be done to cool the generator by cooling the rotor. Referring to FIGURE 30, there is shown a rotor 106 with a hemispherical shape and having field coils 105 placed over the poles. On the hemispherical shape are disposed cooling ridges 107. The ridges 107 are, for example, machined in the rotor 106. The fins 107 are shown in a helix, but parallel fins can also be used. Referring to FIGS. 31 and 32 another embodiment is shown, wherein radial blades 108 are fixed onto a rotor 109 so that as the rotor rotates, the blades function as fan blades. The blades 108 are covered by a cowling 111 which is fixed thereto to increase the air flow over the rotor 109 and to direct an air stream to the field coils and to the stator (not shown). The cowling 111 is in heat conducting contact with the blades 108 which are in heat conducting contact with the rotor so that added cooling surfaces are provided. The rotor 39 for the four-pole generator, shown in FIGURES 9, 10 and 11 could be also provided a cowling and fan blades simi lar to those shown in FIGURES 31 and 32.

In addition or as an alternative, the rotor could be liquid cooled. Referring to FIGURE 33, a generator is shown schematically with two rotors 112 and 113 having two different embodiments of a liquid cooling system. The rotors are mounted on a shaft 114 which is mounted to bearings 116. The shaft 114 includes, for example, a pulley 115 for driving the shaft and rotors. The field coils and stator are not shown for simplicity. The cooling fluid, which may be oil of a suitable type is introduced into an axial duct 117 of the shaft 114 by a tube 118. A suitable shaft seal 119 is provided. The liquid branches into radial ducts 121 and connecting ducts 122 in rotor 112 back to the axial duct 117. Plug 124 prevents the fluid from passing straight through duct 117 while plugs 125 are suitably placed to seal ducts 122. In one embodiment the liquid could flow through similar ducts 121 and 122 within the shaft and the other rotor and out of the shaft through tube 123. As an alternative, the right hand rotor 113 is shown cooled by means of round or fiat tubing 126 brazed to or in other heat conducting contact with the surface of the rotor 113. The tubing 126 forms a network of which only One is shown and is suitably connected to the axial duct 117. This system is cheaper to construct than the internal ducts shown for the left hand rotor 112.

The field coil embodiments so far shown give a maximum flux per pole, but the flux is approximately uniform over most of the pole face and therefore when the generator produces alternating current the output is not sinusoidal. Although, the waveform of the alternating current could be corrected by stator winding considerations, in some cases a more nearly sinusoidal magnetic field would be more desirable. Referring to FIGURES 34 and 35 there are shown a field coil 131 and a rotor 133, respectively which coil concentrates more flux at the center of poles 132 (which are for the two-pole rotor 133) than near the circumferential ends. Instead of one radial slot that is disposed between the poles, there are a number of narrow slots 134a, 134b, 1340, 134d and 134e to accommodate, respectively, a number of coils 131a, 131b, 1310, 131d, and 131e. The smallest coil 131a is placed in the center of the poles 132 within slots 134a, coil 131!) in slots 134b, etc. The coils 131a, 131b, 1310, 131a, and 131e are preferably connected in series to ensure that the greatest density flux is within coil 131a. This system can be reinforced, for very high speed operation by a support such as ring 89 in FIGURE 22.

Having described the novel features for making an improved rotor with electromagnets, the following is a description of the novel features for making an improved stator for the axial gap generator with electromagnets.

Referring to FIGURE 36, there is shown a portion of a startor winding 136 of the prior art which may be incornular insulation board 1 46 which also provides support to the winding. in order to reduce eddy current losses the ribbon from which winding 136 is termed may be l inated. FIGS. 37 and 38 show how a continuous metallic ribbon -147 may be folded for the laminated winding 136. The ribbon 147 comprises a series of alternate lateral offsets 148 and 149 joined by connector portion 151. When the ribbon 147 is folded about lines 162, the result is as shown in FIGURE 38 and when then folded about lines 156 the result is as shown in FIGURE 39, i.e. the ribbon 147 has a zig-zag shape. When the involute curves 137 and 138 are formed therein, the ribbon is properly shaped and the end result resembles the portion of the winding 136 shown in FIGURE 36. This winding is of a desirable type tor the generator of this invention, since it permits free choice of coil pitch and a balance of such factors as wave form and efficiency, and since the winding consists of two sides symmetrical about the insulation board 146, the windings remain relatively cool when in use. However, as observed in FIGURE 3'6'there is wasted space between the straight radial portions 139 of the windings.

FIGURE 40 shows a metallic ribbon 156 which is a modification of the ribbon 147 in FIGURE 37 wherein the waste Space is used. The ribbon 156 is folded first in one direction on lines 157, then in the other direction on lines 158, so that it assumes the shape shown in FIG. 41, then the ribbon is folded on lines 159 forming a zig-zarg pattern, and the involute curves are formed on the folded zigzag ribbon as in the winding 166 shown in FIG. 36. The ribbon is assembled on an annular insulation board similar to board 146 (FIGURE 3'6). Referring to FIGURE 42, there is shown only a portion of a novel stator winding 161 made of the ribbon 156 '(FIGURE 42) which windings utilize more of the available space. F'IGU RE 42 shows how added conductor layers or lamina 162 are place din the windings. In winding 161 (FIGURE 42) the inner and outer involute curves are also in continuous contact. The additional thickness due to three layers in the outer involute curves of winding 161 is beneficial as the ribbon length on the outside of the winding is relatively long, and an increase in the ribbon thickness and, consequently, in the tinal outer diameter of the stator does little harm but does reduce resistance. The change in cross-section of the ribbon does not have to be in steps but could be changed gradually to further decrease resistance. Referring to FIGURE 43, when a. stator 1 64 fora generator 165 is wound with round strands of insulated wire, the wire on the exterior would require less volume than the wires on the interior. Then, the stator 164 would have a shape, as shown, when the wires in the stator are packed together, with the thickness tapering down as the diameter increases. When this is done, the clearance gap area of the stator is increased at the expense of the spaces between adjacent wires in the stator 164. However, there is not much loss in cooling, and the stator has a smooth surface, reducing winda-ge losses. To insure the presence of maximum -flux in the axial gap, the generator may have rotors 167 mounted on a shaft 168 which rotors have poles 169 with pole faces that match the stator 164. Thus the air gap is the smallest near the outer diameter of the generator. Field coils 171 supply the required excitation to the poles 169.

In the before mentioned stator windings 136 and 161 in FIGURES 3 6 and 42 respectively, the stator when assembled in a generator has half of the conducting elements near one pole face and the other half near the opposing pole face. A stator coil can be formed that is similar in shape to the field coil 29 in FIGURE or to the field coils 87 (FIG. 21) or 97 (FIG. 25). Such a winding may be wound to form, or, wound in the most convenient shape, but with the correct means perimeter, and formed afterward. Such a stator coil is taught in the above-mentioned copen'ding patent application No. 529,- 603. That application teaches the use of six stator coils, each having a semicircular shape and disposed in three layers with two coils in a layer. The two coi s in each layer are angularly displaced at with the two coils in the adjacent layer so that a three-phase alternating current is produced. This type of stator coil is easy to manufacture but it develops a wave form which is not a true sine wave when used in connection with a non-distributed field winding. Also, since one of the layers of coils must be in the center, i.e. covered by the other two layers, the center layer does not cool as well, causing some resistance unbalance in one of the phases as well as differential expansion stresses and some warping. Also, the center layer is more distant from a pole face than the two outside layers causing an unbalance in reactances and voltage.

Referring to FIGURE 44, there are shown two stator coil-s 176 for a signal phase generation whose output is substantially a sin-e wave. Each coil 173 comprises, for example, four coaxial coils 17 3a, 173b, 1760 and 176d mounted on a semi-circular board 17 4. The boards have suitable holes 176 for mounting. The stator coil-s 173 are similar in form to field coils 131 shown in FIGURE 34. Thus, when a generator does not have field coils such as coil 131, the generator shown have stator coils such as coils 173 in order to produce a current that has substantially a sine wave. Each one of the coils 173a, 176 b, 1760 and 176d is preferably wound of thin wide ribbon con-ductors having a uniform thickness and Width. The coil-s 178a, 1731;, 173a and 173d in one stator coil 176 are connected in series, while two coils 17 3 may be either connected in parallel or in series to the output terminals (not shown). Referring to FIGURE 45, there is shown schematically another embodiment of two stator coils 17 8 for a two pole generator for producing a substantially sinusoidal current. The coils 17 8 are tightly wound with the radially extending coil elements 179 spaced apart by an angle having a value of approximately 128 or slightly more than /a of a pole pitch. If the generator has four poles the angle should have a value of approximately 64". It should be noted that to produce three-phase current stators made from either coil shown in F-IGUR-E'S 44 and '45, require three levels of such coils.

Referring to FIGURE 46, the figure shows a preferred structure for a three-phase stator 181 for a two-pole generator. On one level, three coils 182a, 182b, 1820, each tightly wound as in FIGURE 45 but having a pitch of 120 electrical degrees, are assembled. On a second level, three more coils 182a, 182a and 1827 are also assembled. Note that the three coils 182a, b and c in one level are disposed at 180 electrical degrees from the respective coils 182d, e, and f in the other level. The angle in the six coils 182a- 182 is 120, less than 128 (mentioned above with coil 178). However, the shape of output waveform is not greatly impaired. The two levels of the six coils 182a 1821 are connected for a balanced three-phase output as follows: Coil 182a in series (or parallel) with coil 182d, coil 18212 in series (or parallel) with coil 182e, and coil 182a in series (or parallel) with coil 182 The efiiciency of this structure is apparent, as the dissymmetry and cooling problems of a structure with three levels of coils are avoided. Obviously more conductors are placed into working position. Naturally, for a given gap width between the pole faces, the thickness of the coils 182a-182j is larger than the thickness of the coils in a three-layer stator. Since only one interlevel insulation is required instead of two, there is actually somewhat more than this ratio in actual available copper space. For phase balance, an accurate relative positioning of the three levels of the prior art stator is necessary. In the stator 181 of FIGURE 46, the requirement is that the six coils 18211-1821 be of the same shape and be symmetrically assembled. A displacement between levels differing from the 180 relationship, even if as much as ten electrical degrees, would cost less than a 2% voltage loss, but the phase balance would not be disturbed. Displacement errors of less than :5 have practically no effect.

The mechanical problems connected with rotor body design, and with the field winding, have been treated. The

13 structures required for the stator support will now be disclosed.

The major consideration here is that the forces on the stator are relatively small. There is, of course, a very large force of attraction between the poles of opposing rotors 25 in FIGURE 4, but this force is all carried in the shaft 28. The force between the field flux and the stator under load is substantially the same as the mechanical torque absorbed to produce the electrical power. In small generators the torque is relatively small, but the torque increases as the size of the generator increases, and the available structure for restraining the stator becomes relatively weaker. This is so because the air gap should be relatively small even in a generator with large diameter. Then, the stator for the larger generators would have a relatively small thickness to diameter ratio, meaning that the stator would be flexible. The basic problem in supporting the stator is that it can be held only on itsoutside' diameter. In this invention means are provided to stretch the stator as a drumhead is sketched so that it becomes stiffer.

Referring to FIGURES 47 and 48, there is shown schematically the stator 161 of FIGURE 42 with stretching means. Only a few of the elements 162, 163a, 163b, 166 and 184 are shown for simplicity but these elements extend 360 around the shaft 28. The stator 161 as mentioned before, has straight portions 162 connecting to involute curves 163a and 163b. The inner involute curves 16% are connected to inwardly extending tabs 166. However, herein tabs 166 have suitable recesses in which rings 183 are inserted as shown in FIG. 48 on both sides of the stator 161. Rings 183 could be made of, for example, of surface insulated non-magnetic strong metal wire. The outer involutes 163a connect to outwardly extending tabs 184. The tabs 184 are cast in, for example, an epoxy compound forming a plastic ring 186 around the stator 161. The plastic ring 186 is machined as shown in FIG- URE 48 with opposing grooves 187 which have their outer surfaces conical as shown. The plastic ring 186 is preferably cut into a plurality of segments 188 (FIG. 47) for reasons that will become apparent hereinafter. Two continuous rings 189, made of, for example, strong nonmagnetic metal, are disposed to engage the plastic ring 186 or more specifically the segments 188. The rings 189 have projections 190 which match the grooves 187 on segments 188. Then when the rings 189 are bolted to a support 191, the conical surfaces in grooves 187 and on projections 190 equally force the segments 188 radially outward placing the stator 161 under tension.

Another method of tightening a stator is shown in FIGURE 46. Therein a ring 193 of strong material is disposed around the six coils 182a-182f. Another ring 194 is disposed on the inside of the coils. Between the openings of the coils a strong wrapping 195, for example, glass filament, is wrapped as shown around the coils and rings 193 and 194 for one of the six openings. The wrappings for the other five openings are not shown for clarity, but when these wrappings are applied the coils would be in tension especially if the outer ring 193 is slightly oversize.

Various other modifications and variations of the present invention are contemplated and will become apparent to those skilled in the art without departing from the spirit and scope of the invention. Therefore, the invention is not limited to the exemplary apparatus or procedure described but includes all embodiments within the scope of the claims.

What is claimed is:

1. An electric generator of the axial air gap type comprising:

a rotary field producing structure having a rotor disposed on each side of said air gap with at least one N-pole and at least one S-pole on each of said rotors, with said N-pole on one rotor disposed facing said S-pole on the other rotor;

said rotors being disposed to rotate together about an axis normal to said gap,

an electromagnet coil disposed around each of said N-poles and said S-poles to produce the required magnetomotive force, and

a stationary stator disposed within said air gap to produce an electric output when said rotors rotate. 2. The generator of claim 1 wherein: each of said rotors has only one N-pole and only one S-pole,

each of said rotors having generally .a cylindrical shape with the two poles protruding from one end diametrically opposite to each other, and the other end thereof having a semicylindrical surface whose axis is disposed normal to the diameter passing through both poles and to the axis of the rotor.

3. The generator of claim 1 wherein said poles are shaped to have maximum flux density at the pole face to compensate for magnetic flux leakage.

4. The generator of claim 1 wherein each of said rotors has at least two N-poles and two S-poles with the N-poles disposed between two S-poles and the S-poles disposed between two N-poles,

each of said rotors having a frusto-conical shape with said poles disposed on the larger diameter end of the rotor.

5. The generator of claim 4 wherein the smaller diameter end of said rotor has a well formed therein with substantially sloping sides and valleys extending radially which valleys are respectively disposed opposite said poles.

6. The generator of claim 1 wherein a reinforcing means is wrapped around said coils for holding the centrifugal force produced by said poles as said rotor rotates.

7-. The generator of claim 1 wherein said electromagnet coils are formed of ribbon conductors spirally wound one layer over the preceding layer around the respective poles with the larger dimension of the conductor extending axially.

8. The generator of claim 1 wherein said rotors have removable poles.

9. The generator of claim 8 wherein at least one of said rotors has buttress-type circumferential groove concentrically formed on one end and said poles have buttress-type grooves formed at one end to match the buttress-type grooves on said rotor, and

means for attaching said poles to said rotor.

10. The generator of claim 8 wherein a pole face damper made of conductive metal is fixed to said poles.

11. The generator of claim 7 wherein said ribbon conductors are Wider in the outer circumferential region of the rotor than in the region between poles.

12. The generator of claim 1 wherein said electromagnet coils are formed of ribbon conductors helically wound around said respective poles with the larger dimension of the ribbon conductor extending radially.

13. The generator of claim 12 wherein said ribbon conductors are wider in the outer circumferential region of the rotor than in the region between the poles.

.14. The generator of claim 12 wherein some of the layers of said ribbon conductor are wider than other layers in the outer circumferential region of the rotor to provide more cooling area on the coils.

15. The generator of claim 12 wherein the layers of the ribbon conductors in the outer circumferential region are1 spaced apart to provide more cooling area on the CO1 s.

16. The generator of claim 1 wherein the shape of the poles is such that the poles are stepped at the interpole slot formed between two poles on the same rotor.

17. The generator of claim 16 wherein said electromagnet coils are formed of ribbon conductors spirally wound in one layer over the preceding layer around the respective poles with the width of the ribbon conductor within the region between poles being made larger in the outer layers than in the inner layers in order to have the coils fit closely around the stepped poles. 

